The present invention relates to a torsional damper for absorbing peak engine firing impulses normally transmitted from the engine to the drive train in a marine drive unit. More particularly, the invention relates to a torsional damper which utilizes a highly viscous, high shear fluid to provide a low slip fluid transmission of torque between the engine crankshaft and the driveshaft while absorbing peak torsional vibrations.
As is well known in the art, the use of an internal combustion engine to provide motive power results in torsional vibrations which are transmitted to the engine drive train as a result of the sequential engine firing impulses. The peak engine firing impulses induce very high stresses on components of the drive train resulting, in many cases, in reduced component life. In a marine drive unit, such as an outboard motor, peak engine firing impulses are known to induce particularly high stresses on the gears in the lower gear case which translate rotation of the vertically disposed driveshaft to rotation of the horizontally disposed propeller shaft. High vibrational stresses are also known to be imposed on the elastomer member used to mount the propeller hub on the propeller shaft.
Two principal means of reducing the transmission of torsional impulse stresses from an engine to the drive train in a marine drive unit are to utilize a heavy engine flywheel or to provide a rubber or other elastomer coupling between the engine crankshaft and the driveshaft to the drive unit. The added weight created by a heavy flywheel is particularly unattractive in a marine application and, in the case of an outboard motor in particular, the typical location of the flywheel on top of the engine makes it unsuitable for absorbing torsional vibrations in any event. In a stern drive marine engine, the use of a heavy cast iron flywheel to absorb torsional vibrations results in both a weight and space penalty. A stern drive may utilize a rubber coupling to supplement the vibration damping provided by the engine flywheel. Because of the relatively harsh environment in which marine drives are often operated, deterioration of rubber or elastomer couplings detracts substantially from their utility for use as torsional vibration dampers in both outboard and stern drive application.
Nevertheless, the damping of torsional vibrations resulting from peak engine firing impulses is needed to maintain reasonable life in the gear set used for the transmission of power to the propeller. With respect to the propeller mounting itself, an elastomer mount with greater stiffness may be used to better absorb high torsional vibrations. However, a stiffer rubber or elastomer propeller mounting exposes the propeller to potentially greater stresses imposed by external forces acting directly on the propeller.
One type of viscous fluid coupling, used as a limited slip differential between axles in four wheel drive vehicles, utilizes a closely spaced stack of plates which are alternatively attached to the relatively rotatable components of the differential and between which a viscous fluid provides the driving connection with a limited amount of slip. This application, however, is not concerned with the damping of torsional vibrations and the relatively large amount of slip provided by this coupling would be unsuitable in a marine drive unit application.
Thus, there continues to be a need in marine drive units, both outboard motors and stern drives, for an improved means of absorbing torsional vibrations and eliminating their transmission to the drive train of the unit, thereby enhancing drive train durability and life. Any torsional vibration damper should be resistant to deterioration as a result of the environment in which it must operate and, preferably, should provide a weight and space saving over present devices utilizing an engine flywheel for damping. The device must also be capable of uniform and consistent operation over a wide range of temperatures typically encountered in the operation of a marine drive unit.